Devices analyzing the structure of stratified objects by means of ultrasound are already known, such as for instance from the EP 0 262 186. The known devices are based on the principle of reflection tomography. This principle applied to fingerprints according to the mentioned European Patent was presented at the Conference of Acoustical Imaging in Florence in 1995 and published in the respective Proceedings under the title "Ultrasonic setup for fingerprint pattern detection and evaluation". In the known references, the object placed on a support plate is subjected to ultrasound waves coming from a generator arranged oppositely thereto in a liquid filled housing. Either several generators and several receivers are arranged on the housing wall opposite to the support surface, or the generator and also the receiver are movable on a carrier along a trajectory. In this way the finger tip or any other object on any point of the support plate can be detected. The waves reflected and back-scattered by the object are received by a receiver which transmits the information about the intensity and/or the phase of backscattering and reflection to a computer for analysis and recording, optionally via an amplifier, a timing device, possibly also a detector.
The ultrasonic waves to which the finger or other objects are exposed are bulk waves originating from the generator/generators in the same manner as the backscattered and reflected sound waves, which are in the range of 2 MHz and above.
The production of devices of the known kind has proven to be complicated, particularly because the housing is filled with liquid. This requires the housing to be perfectly sealed off and the arrangement of the transducers in the liquid is also fraught with problems. Furthermore due to the liquid-filled housing and the use of bulk waves, these devices are also relatively large, since a certain size of the device can not be reduced. They also do not allow for a large size of the support plate, which makes impossible the analysis of objects with a large surface, e.g. human hands.
Further devices used purely for position detection have become known. Such a contact sensor is described in the abstract of JP-A-2 195 289. The latter consists of a tube serving as a wave guide, at whose open end an ultrasound transducer serving a sender, or receiver, to which a transmitting and time-measuring circuit, as well as a distance calculator are connected. Depending on the mechanical pressure on the wave guide, the reflection of the ultrasonic wave is determined and therewith the pressure point is calculated. A structural analysis can not be performed with the device of the aforementioned application.
There are also other known device which detect the position of an object, such as the position of a finger on a surface, e.g. a display (see also EP 0 557 446 and 0 523 200). However these devices are not capable of analyzing the structure of the object, e. g. the finger, since they have poor resolution and also because they are not intended for this purpose. Also their modus operandi is different, since these devices use only the attenuation of the ultrasonic waves which are generated by the object, which in principle can not give information about the structure.